Measurement of prestressing force with a partly destructive method
For the assessment of existing prestressed structures such as bridges, beside the knowledge of the material properties, amongst others, also the actual prestressing force in the tendons has to be known. Within the framework of this project, on an old bridge girder, which was internally post-tensioned with two tendons consisting of parallel wire bundles and several pre-tensioned wires, the force was locally measured by means of a partly destructive test method. The test method is introduced and discussed.
Investigation of post-strengthening using strips made of carbon fiber reinforced polymer
In the case that existing structures such as bridges or buildings have to be strengthened, because they are in bad condition or due to the existence of new higher loads, then, the use of carbon fibre reinforced polymer (CFRP) is a good possibility. This material has the advantage that it is very light and can be applied manually. Furthermore, it has high tensile strength in longitudinal direction and high fatigue strength.
1. Evaluation of prestress load.
2. Determination of shear transfer capacity of middle joint and comparison with loads according to SIA 160 acting on existing bridge.
3. Design of CFRP flexural strengthening using various design approaches and comparison with test results.
4. Comparison of CFRP flexural strengthening bonded in untensioned and tensioned state.
5. Health monitoring using dynamic methods.
6. Post-strengthening of damaged beam with prestressed plates.
Five beams of the "Viadotto delle Cantine a Capolago" highway bridge will be transported to Swiss Federal Laboratories for Materials Science and Technology.
The following investigations are planed:
Evaluation of prestress load: Wires of all five beams will be partly uncovered, individual wires cut and strain measurements taken. The loss of prestress will be determined by comparison of the original and remaining prestress loads.
Shear transfer of middle joint: One problem with the existing bridge is that there is no untensioned reinforcement running through the mid-span joint and it seems that the joints are partially opened. The test is designed to show the magnitude of the shear loads that can be transferred. This can be compared with the effective shear loads acting on the existing bridge.
Flexural strengthening with CFRP (untensioned): Flexural strengthening by means of CFRP plates bonded to beam soffit in untensioned state. Delamination, the most common failure mode, represents a major problem with CFRP flexural strengthening of reinforced concrete beams. The ultimate load will be calculated in advance using various design approaches and compared to the test results.
Flexural strengthening with CFRP (tensioned): Flexural strengthening by means of CFRP plates bonded to beam soffit in tensioned state. The prestressing system will be anchored using the "Meier / Stöcklin gradient method". Prestressing is expected to improve performance under serviceability and ultimate limit state in comparison to the untensioned variant.
Health monitoring using dynamic methods: The deformation behaviour and load-bearing capacity of the beam is strongly influenced by the existing prestress load in the middle joint. The cutting of individual wires in the prestressing tendons at mid-span, in each case with subsequent experimental modal analysis, will help determine whether the prestress load can be reliably monitored.
The beam will then be strengthened by means of prestressed CFRP plates. The effectiv prestressing conditions in the beam will be determined using dynamic methods.
The strengthened beam will be subjected to a static loading test. The aim is to clarify whether damage to the CFRP strengthening (delamination) can be detected using this method.
Final report (attached).